It’s easy to see why Moscow backtracked. The rouble was down by more than 6% against the dollar in early trading and is vying with the Ukrainian hryvnia as the world’s worst-performing currency of 2014. The word “freefall” is much overused in financial markets. Not this time.

More than two-thirds of Russia’s exports are from the energy sector, and the cost of crude has dropped by 40% since the summer. The sell-off has accelerated since Opec decided against production curbs at last week’s meeting. Downward pressure on the rouble intensified amid market speculation that there would be no intervention.

There were reasons for adopting a hands-off approach. Chucking a chunk of Russia’s sizeable reserves at a rapidly falling rouble could be an expensive and ultimately pointless exercise. Intervention is normally more effective when it is going with the grain of market sentiment, and the rouble is currently toxic.

What’s more, a weaker currency should help to support growth and take some of the pressure off the budget by boosting the rouble value of Russia’s oil reserves.

So why did Moscow blink, given that the central bank’s official line has been that it would only step in to defend the rouble if there was a threat to financial stability?

That, though, is precisely the point. A rouble in freefall does pose a threat to financial stability in two big ways. First, it increases the foreign currency value of Russia’s foreign liabilities, worth about $200bn (£127bn). Second, a continued fall in the exchange rate will encourage Russian citizens to convert roubles into dollars and euros, thus increasing the risk of bank runs.

Russia’s banks are in better shape than they were in 1998 and the government has the financial firepower to help them out should they get into trouble. But a plunging oil price and a plunging rouble still make the banks vulnerable. Hence the need to intervene.

All the latest business and economics news, as the Russian economy ministry said it expects GDP to shrink by 0.8% next year amid more capital outflows and higher inflation, sending the rouble sliding again